As is known in the art, it is frequently required to connect a microwave transmission line formed on one structure to a microwave transmission line formed on a second structure. For example, one structure may be a printed circuit board having coplanar waveguide (CPW) or microstrip transmission lines and the other structure may be a monolithic integrated circuit (MMIC) chip having active and passive devices interconnected with microwave transmission lines. In some applications, one or both structures may be printed circuits or one or both may be MMICs. In any event, the microwave transmission line at the output of one of the pair of structures may be required to be electrically connected to the microwave transmission line at the input of the other structure.
As is also known in the art, because of: Mismatched difference in the Coefficient of Thermal Expansion (CTE) of the two substrates, the two structures are mounted with a small gap, typically 5-15 mils, between them to prevent stress that can crack or damage the devices on the MMIC chip; Automatic assembly techniques typically place the two structures to a specific true position with some level of accuracy; however; there is a placement tolerance on the vision accuracy and the accuracy of placement due to motors, linear encoders, etc., in addition to allowing for substrate size tolerances on the structures; minimum gaps may also have to be created to meet assembly tool in automatic assembly apparatus such as pick and place collets (where flat face tools cannot be used), wire and ribbon bonding tools and dispense tools that all have their own specific access requirements. Therefore, one technique used to solve these problems is to place the opposing sides of the two structures as close together as possible and then use, as the electrical interconnect, wire or ribbon bonds, as shown in FIGS. 1A, 1B and 1C.
As is also known in the art, in many applications it is required to isolate the electrical interconnect from radiating to other parts of the circuit or being influenced by signal from another part of the circuit. As noted above, a traditional method for interconnecting is by wire or ribbon bonds; however, wire bonds are notorious for radiating signal. Further, if only the signal conductor is bonded, there is a very large amount of microwave radiation emitted from the bond. If ground signal ground bonds are used (as shown in FIGS. 1A-1C), the radiation is reduced but is still fairly strong and may be unacceptable in many applications and therefore, depending on the application, this can cause undesirable feedback oscillations or feedback induced ripple.